Clickable links in event descriptions

[igordsm]

Fix #571 . Implementation was based on this mailing list message. Requests for improvements are welcome.

[mcclurgm]

Thanks for the PR! It seems to work as advertised for me. I haven't taken a very close look at the code, but in the meantime there are a few features that could use some work here:

1. The formatting of the link should follow the system theme. I'm not sure how to get this to work in a TextView, but it should use the current accent color and look good in both dark and light modes. @elementary/ux do we want to use the system accent color for links like this or do we want to use the green accent that we use elsewhere in Calendar?
2. There should be some sort of tooltip to indicate that links are clickable. Include something like "Hold ctrl and click to follow link" (the copy could use some work, of course).

[danirabbit]

I'm gonna convert this back to draft since there hasn't been any movement in the last couple weeks. Feel free to mark ready for review once requested changes have been made :)

[igordsm]

I'm waiting on the granite update to proceed. As soon as it lands I'll get back to this.