Policy changes: Adjusting Freezes

[David Edmundson]

Notes from the plasma meeting:

Freeze proposal:
    - Feature freeze 2 weeks before beta tagging (just before the
relevant frameworks tag). String freeze remains on beta day as before.
Branching remains on beta day as before.
    - Effectively this is formalising the existing unwritten rule of
not merging big changes at the last minute.

After 5.19 we will review this and potentially make adjustments.