Add link search and backend notes field.

Here are some other pieces to the link-management puzzle. In this pull request:

1. Add a search form to the list of links. If we're inside a folder, only that folder is searched.
1. Add a freeform `notes` textarea to each link (visible only to users who created/vested the link). This will be searched, so we get a weak form of tagging for free (you can add "needs_review" to the notes field and then search for that), along with the other things users could use a notes field for.
1. Remove pagination, which shouldn't be necessary now that we have folders, and squish down the link display to two lines so more links can fit on screen.

**Please note:** We ended up with two migrations named 0007_ as the result of a merge. I deleted those and created a single 0007_ migration that incorporated both of them, and then added a 0008_ migration for the new notes field. Pay attention to how this works on staging before taking it live -- I'm not sure whether I properly resolved the conflict.