generalize profile links

[cellio]

Fixes #1248.

Adds user-controlled extra fields to the profile to replace the baked-in website and twitter slots. You can put whatever you want here; proper URLs get linkified and everything else is text, like so:

Edit profile section:

Main profile view:

The same link-nerfing rules that applied before apply to this new section.

Added a maintenance task to initialize new table rows for all current users. After discussion, we decided this would be better than constructing new rows on demand (and creating the "add" UI in the editor). When the task is run, all users will get three entries in user_websites, with the first two being filled with the website and twitter links if they exist. This allows the profile editor to present a form with three label/URL pairs for editing. For new users, we add these rows after creation.

Out of scope here (could be considered in the future):

• After this safely deploys without issues, delete the old website/twitter values after migrating. (TBD: maintenance task to remove them or schema change to drop the columns.) Remove from authenticated_user_object and configure_permitted_parameters as well (mentioned in chat by Oaphi).
• Make the number of websites configurable in a global site setting. Handle the potential data loss if the limit is lowered (the main reason we're not doing this now). My recommendation would be to preserve the data and limit the number that are shown. (In the editor, we'd show the extras in some grayed-out-like state, so that users could still recover that data if they wanted to move it up.)
• Better integration w/third-party platforms, for example generating the icon and link like we did for twitter before this change. (I'm concerned about maintenance burden and complexity here, but acknowledge that entering a proper link vs. just a username could be a stumbling block for some users.)

Deployment note: After deploying, visit /maintenance_tasks, click on the initialize task (the only one), then click on "run". With a large number of users we might need to do that more than once, so check the output. (I believe the task times out after 5 minutes.)