We no longer use all the alert words for all the users in the
entire realm when we look for alert words in a newly sent/edited
message. Now we limit the search to only all the alert words
for all the users who will get UserMessage records. This will
hopefully make a big difference for big realms where most messages
are only sent to a small subset of users.
The bugdown parser no longer has a concept of which users need which
alert words, since it can't really do anything actionable with that info
from a rendering standpoint.
Instead, our calling code passes in a set of search words to the parser.
The parser returns the list of words it finds in the message.
Then the model method builds up the list of user ids that should be
flagged as having alert words in the message.
This refactoring is a little more involved than I'd like, but there are
still some circular dependency issues with rendering code, so I need to
pass in the rather complicated realm_alert_words data structure all the way
from the action through the model to the renderer.
This change shouldn't change the overall behavior of the system, except
that it does remove some duplicate regex checks that were occurring when
multiple users may have had the same alert word.
It appears that the assertRaisesRegexp approach we had before didn't
work properly on some systems, likely due to a bad interact with a
i18n (we haven't definitively determined the cause).
We now raise an exception in bugdown.do_convert() if rendering
fails, to avoid silent failures, and then calling code can convert
the exception to a JsonableError.
Since we don't have a stable way to get the Dropbox preview failure
image (and it was sorta a weird setup anyway), it seems best to just
remove the condition.
