Add "About me" field to user profile and make it visible wherever relevant

[tomnixon]

Add additional free text field to user profile called "About me" where users can give more info about themselves such as their username for Slack, contact details, and any other info they wish to share.

Data can be edited by the user from their profile page when they click on their avatar in the top-right. The extra field is labeled "About me" and is shown below email address. There is a note underneath it to show that markdown can be used (in same way as it's displayed when a user edits the circle description).

This info will be displayed as an extra tab labeled "About me" on the profile page in the directory, after the "Contributor" tab. The Lead tab will still be the default. On this tab, the about me markdown will be displayed.

If the user is looking at the directory page for another user and there is no about me data, then display a message: No "about me" information to display.

If the user is looking at their own directory page then display this message about any markdown Edit my "About me" information. This text will be a link to edit their profile.

[rgoj]

This is quite a big issue, we probably need to work out exactly where to make use of this field and how...

...but the first component of this is relatively obvious, so I separated it into its own issue: #668

[rgoj]

We just worked out a solution for this that ticks all the boxes, I think! It makes the UI better for everyone - even those who won't be using the feature and it solves the problem reasonably well for those who will be using it.

1. We'll name the field "about" so that it works in all places (rather than "about me" which won't work on the settings page). Maybe we could later change it to something like "profile text" for example or something better if we come up with it. But for now we settled on "about."
2. We'll only show it in the directory and make it editable in the member form (which is used in 3 places), not worrying about any other locations were people might want to see this information for now.
3. In the directory, we'll move the "image + name" part of the page up to separate it from the "circles this person is in" functionality of that page. This will make it more obvious for everyone what is happening on the page and make room for the new field.
4. For people with the "about" field filled out (and only for them), the text from the field will show up underneath the "image + name" heading and before the "circles" section.
5. There will be no "about" heading for people without that field filled out, no "Edit about" hint to make people add that information or anything like that - to make this cleaner for those people who won't want to use this functionality. This'll make it less discoverable for those who do... but that's a question for combining the directory with the list of people under settings, really! Maybe we could add an edit button somewhere on the directory linking to the settings page if that gets too confusing...?

That should hopefully work UI-wise.

We also discussed the data problem that I missed when writing up #688 . Ideally the "about" field should be per-organisation functionality. And it could be, but that is going to be more complex. To simplify things we will just make it a per-user field, meaning people across different organisations will have the same information there across those different organisations. That's going to be a feature from some perspectives and a nuissance from others, depending on how people end up using that field, ideally we'd have both types of fields, really. But it's probably the best compromise for now.