New scripts to facilitate releases and merging upstream branches

[jason-famedly]

A rework of the original make_release.sh script and a new merge_upstream.sh script, separating these interests.

Switch to a merge instead of a rebase of changes between Famedly and upstream repository's.

Release branches for Famedly going forward will be formatted as famedly-release/vX.Y instead of reusing the release-vX.Y format which matches upstream. This was fine before when rebasing was the go-to action. Now it is confusing and doesn't work cleanly.

The new merge_upstream.sh script should now be able to handle several common scenarios in a smart way. From the comments in the file itself:

# Three scenarios are handled by this script:
# 1. New minor version(updating from `v1.123` to `v1.124`)
# 2. Existing minor version with an upstream patch increment(updating from `v1.123.0` to
#    `v1.123.1`)
# 3. Existing patch version with a local Famedly level patch increment(updating from
#    `v1.123.1_1` to `v1.123.1_2`)

It is to be invoked with a single argument, the major and minor version of Synapse that is to be merged in. Use the format vX.Y for the argument. For example:

./merge_upstream.sh v1.128

Using various tags between our repo and upstreams it will determine that we have no release branch of our own(either locally on repo or remotely on Github), merge in the upstream branch for that version. If there was no changes to the upstream branch since the last time it was merged in, then it will just have the changes from our main branch.

Afterwards, it will (optionally) handle editing the CHANGES.md file for you, by selectively checking for Famedly exclusive changes which will be added to the bottom of the most recent Synapse version change(but above any pertaining 'release candidate' versions).

It will then recommend the tag to use for the release.

The make_release.sh script still expects an argument in the format vX.Y.Z_A for use in tagging the release