Add support for auto-updating of Firefox testing builds

[toasted-nutbread]

https://stackoverflow.com/questions/40541561/firefox-web-extension-auto-update-unlisted-and-self-hosted-extension

[toasted-nutbread]

@FooSoft This would likely require doing one of the following:

1. Maintaining an updates.json file on https://github.com/FooSoft/foosoft.github.io. E.g. https://foosoft.github.io/projects/yomichan/updates.json or https://foosoft.net/projects/yomichan/updates.json.

2. Maintaining an updates.json file on an orphan branch of this repository which is visible as a gh-pages URL. E.g. https://foosoft.github.io/yomichan/updates.json or https://foosoft.net/yomichan/updates.json.

The release process would be:

• Upload releases to AMO and Chrome Store as usual.
• Upload the release files to the release GitHub release page.
• Update the updates.json file with the URL from the GitHub release page.

We could have a checklist template that is pasted into the release issue to help remember what the process is.

Authoring security concerns

• If the updates.json file would be located in this repository, that would likely mean that any collaborators have the ability to modify it without going through a PR.
• Additionally, it may mean that other files on the gh-pages branch could be created on https://foosoft.net/projects/yomichan/.
• For the release files hosted on the GitHub release page, these files can be uploaded/deleted/replaced by collaborators.

So I guess the takeaway is that supporting auto-updating using this repository directly assumes that collaborators are trustworthy and won't "secretly" change the update URLs or download links.

Other

Technically, it may be possible to use a raw URL (on https://raw.githubusercontent.com) for the updates.json file, but the Content-Type header is always text/plain, instead of application/json, which may cause it to not work. Untested and the documentation isn't clear on that point.

[FooSoft]

I think that hosting it on foosoft.net is probably the most straightforward way to go (option 1).

I'd change the URL to https://foosoft.net/projects/yomichan/dl/updates-testing.json, however as the files in the yomichan directory are mainly resources for the HTML there. Explicitly calling out the "testing" also seems like good idea, since Mozilla has already kicked Yomichan off their store once, and I would not be that surprised if at some point the extension has to be hosted outside of their ecosystem.

Speaking of testing releases, are we in a good state to queue up a new one?

[toasted-nutbread]

Sounds good to me.

Explicitly calling out the "testing" also seems like good idea ...

You can also use one updates.json file for multiple extension IDs.

Speaking of testing releases, are we in a good state to queue up a new one?

Most of the features are done for the release, and I'll be doing mostly testing for the next several days. I may do a bit of documentation updates/file organization as well. Right now a lot of new stuff is just dumped in /fg/, /bg/, or /mixed/, and there are probably some ways to clarify the organization.

[FooSoft]

Cool, did not know that you can put multiple extensions in one updates.json, I guess that solves that issue.