Allow granular control for wiki page editing permissions

[llogan]

The trac wiki should allow some pages to be edited only by a "developers" group. This will be useful for some pages, such as the next GSoC ideas page, where last minute vandalism could be a valid concern and would allow us to move away from a third-party wiki. This appears possible as described in TracFineGrainedPermissions but is not a trivial/lazy change.

[michael]

This is fixing a problem we did not have yet.

Also about the gsoc pages, the hypothetical attack is unlikely as the attacker does not know when google would look at the page, thus how should that attack work ?