Kick off ModernBB 2.5 development

This will happen in the 2.4-branch. We also change the way we release
betas and rc's, instead of being called 2.5-beta.1, 2.5-beta.2 and
2.5-rc, we give them a normal version number in the 2.4 branch
(2.4.3-dev for example).
#80 Replace Bootstrap v2.3.2 with v3.0.0-rc1

This seriously damages Backstage, this damage will be fixed during this
ticket and the Backstage redesign. Also: using unstable releases of
Bootstrap can be excepted during the beta stage of ModernBB, however, IF
ModernBB 2.5.0 should be finished before Bootstrap 3.0.0 is, we will
release it with an unstable Bootstrap release and fix that issue so fast
as we can in v2.5.1.