Always store full URLs for redirects

[fklein-lu]

The redirect storage in its current form is not consistent:

• The From field needs to be a relative URL (path and possibly query arguments). Full URLs will not be matched, as per #16.
• The To field can be a relative or an absolute URL. Relative URLs will be prefixed with the home_url().

We should switch to the following:

• Both URLs should be stored as absolute URLs.
• The UI should indicate the domain of the Form URL, by displaying the home_url() before the text field. If we're on a single si
• The URL saving function should ensure that only URLs with the home_url() are allowed in the From field.
• The UI should indicate the domain of the To URL. If there are more domains whitelisted for redirects, a dropdown should be displayed.
• The URL saving function should ensure that only whitelisted domains are allowed to be saved to the database.

[roborourke]

Given the redirect matching is done on the path why store the full URL in the "From" field?

Storing the full URL in the "To" field adds a step to migrating a site to a new domain too. Perhaps not an issue if you'd need to do a search-replace anyway but I'm not sure I follow what the benefit is to the proposed data storage solution.

Big +1 for validating using the whitelisted domains.

[drzraf]

Rather against this: Storing the full "To" URL would:

• Hardcode the domain name thus making local-intended/specific redirects unmovable from one domain to another
• Loose (or make harder) their distinctive nature. (Not able to add a trailing-slash needed for WP local URLs)