Rebase Base Browser to 115 nightly

For ESR 115, we want to gain a couple of months by starting to rebase once 115 hits nightly.

It will involve a few more rebases, but I expect the most of the problematic changes to be from 102 to 114, rather than being on 115 nightly-115 stable.

Also, this will give us a time window on which we are on par with Mozilla, and we can use it to get a few changes upstream.

Last year we saw that the most problematic changes were in the Tor Browser part of the patch set, and we think that splitting the rebase in Base Browser and Tor Browser makes sense and will help the review process. The issue for the Tor Browser part of the process will follow.