Allow extensions to specify a load priority

[KyrneDev]

This would allow a simple fix to #1832 as well as other load priority problems. I have confirmed that after using these proposed changes + adding a priority to the lock extension the issue went away.

Changes proposed in this pull request:
Allow extensions to specify an optional load "priority" in their composer.json under Flarum extension that dictates the order they are loaded in (higher means first)

Reviewers should focus on:
Is this the best way to do this?

Confirmed

• Backend changes: tests are green (run php vendor/bin/phpunit).

Required changes:

• Extensions such as lock need a priority set

[dsevillamartin]

Most likely, down the line, we'll want to implement a more robust system that avoids issues like this without using mere numbers, ex by specifying if the extension has to load before and/or after extension(s).

For now though, I guess this should be fine, and it is similar to the initializer logic on the JS side.

[franzliedke]

I'd like to think about this some more. If I have no better idea, we can merge it before the next beta release.

[franzliedke]

I cannot say I like this very much, because it does not tackle the root cause (the tags extension overwrites the fallback permission check, lock adds an additional performance permission "middleware", so to speak - maybe we should differentiate these concepts?).

I can revisit this when creating an extender for the permission system.

Leaving for @luceos to review and merge.

[luceos]

I dislike this approach too, it bloats the composer.json file unnecessarily. I rather go with @franzliedke proposal right now and do this properly from the get go.

Sorry, but this feels like one of those PR's that might haunt us in the long run.