Move create forum post and share forum buttons in action bar

According to the designs in #305, the 'create forum post' and 'share forum' buttons will go into the in action bar.

In the spirit of keeping MRs small and to avoid conflicts with #306, I did this first and independently of future work.

This also introduces constants for the `GROUP_ID`, `FORUM_NAME` and `MIN_TIMESTAMP` as they were used for the two buttons.

Closes #313

See merge request !150