Starting to redo navigation; users can star questions

Navigation and searching are becoming impossible. This commit starts the process of changing the question index page, so the main index has a few short lists of questions to go to and a search box, and the main listing is on a separate search page.
This also adds a "favourite questions" list per user, which is displayed on the question index.
lots of rejigging of question list views

search and favourites views inherit from a common template which provides the table of results.
news view is now a subclass of TemplateView so it can set the active tab in the rendering context.
editors' picks

Editors (users with the permission editor.question_highlight) can highlight questions, which makes them appear on the front page.
editors' picks page; rejig question edit view; "nothing here" text
Merge branch 'master' into organisation
custom tag model with 'official' field; tag admin actions

Tags can be marked as 'official', which means they will show up on the front page tag cloud (once I've made it)

This also adds two admin actions, "mark as official" and "merge tags". When tags are merged the tag which is most-used is kept. You can always go to that tag and change its name afterwards.
Merge branch 'master' into organisation
fix tag migration
tag cloud on the question index

shows official tags. Size is assigned based on how many questions use the tag.
show tags in header on search page; remove input box

The tag input on the search form was worse than useless because it put people off writing in the search box, which also looks at tags.
This displays the tags being searched for in a header above the search box; doing another search removes the tags query.
Merge branch 'master' into organisation
exam stars/picks/etc.

plus a load of styling, I think.
make the difference in font sizes in the tag cloud smaller
Merge branch 'organisation'
make highlight permission description <50 chars
vertically align star with name text
rejiggle index pages

"log in to create exams/questions" message in place of create link when not logged in.
Split editors' picks and starred/recent things between the two columns, so they're more balanced.
Tags now "popular topics" and on their own row above the question lists.
Show user's recently edited questions on the index as well as starred questions.
Search box now has some big text outside it instead of inside as placeholder text.
<p> round log in reminder on index
typo: very -> every
log in message on index page is a hint