branches: add esr branch support (bug 2048638)

[Magnolia-Liu]

This change makes the CLI + GUI recognize the mozilla-esr / comm-esr / esr pattern and construct the hg repo URL dynamically.

Resolution is a blind HTTP fetch: we build the URL from whatever version the user passes and request it. If that ESR exists on hg the fetch is a hit and bisection proceeds. If it doesn't, the request simply misses and the branch is treated as unavailable.

Previously, mozregression was not working as expected since it could not find any builds when using commands like mozregression --app thunderbird --repo comm-release for Thunderbird branches other than comm-central. This was because comm-esr, comm-release, and comm-beta don't have opt builds, they only have shippable ones.

Fix the behaviour for fetching integration builds both for comm-esr branches and other comm branches by implementing similar behaviour as existing tk_routes construction in Firefox.

Add small help text that shows how to fetch ESR branches as they will not show up in the drop down menu.

New UI:

[github-actions]

Coverage is 90.782% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[github-actions]

Coverage is 90.782% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[github-actions]

Coverage is 90.782% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[github-actions]

Coverage is 90.782% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[coveralls]

Coverage is 90.997% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[coveralls]

Coverage is 90.997% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[coveralls]

Coverage is 90.997% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[coveralls]

Coverage is 90.997% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[coveralls]

Coverage is 90.948% — magnolia/bug2048638-add-esr-branch-support into main. No base build found for main.

[janbrasna]

FYI mentioning previous art in #1891 or f37208c in case there's any inspiration. (E.g. the tag_of_esr() bit there might be interesting?)