Every page constructs a large block of attributes for a RB.UserSession

instance, which is used to let JavaScript code determine session-related

information. This has historically been done in the template itself,

which is slow, hard to maintain, error-prone, and wastes a lot of

whitespace.
This change introduces a new {% js_user_session_info %}` template tag,

which does the work instead. It's much faster and easier to extend, and

cuts down quite a bit on what the template has to do.
It also handles defaults better. The system-wide rich text setting is

now used if a profile isn't yet set for the user. Timezone offsets are

also based off the configured timezone and not the page render context.
Besides that, none of the actual content has changed, and we're

leveraging the benefits of the json_dumps template filter, giving us

more reliable values going forward, if we start needing more

information.

Compared the output of the old and new payloads (normalizing both so

they could be easily diffable) for both authenticated and anonymous

users, and verified that the data remained the same.
Unit tests pass on all supported versions of Python.