Store preferred language for resources (closes #93)

[mattstauffer]

Replaces/continues #94, closes #93 -- moved modifications to that PR into here so it could be a branch I have access to.

@Treggats, I started work on revising your PR. I paused when I saw that you were storing the user preference as an int; I'd rather we store them as strings. I'm going to pause for now, but next steps as I see it:

• Change user preferences to be stored as strings
• Move preferences to be a JSON column on the user. Consider something like https://github.com/Grimthorr/laravel-user-settings (but maybe it's better to just write it ourselves?)
• Matt review the preferences page
• Add the preferences back to the module page, as per my original mockups, and build them in Vue
• Add tests around PreferenceController
• Fix for non-logged-in user
• Add the logic that merges user settings and session settings when it comes to resource language
• Work through the insane number of @todos I introduced
• Decide: should the resource switcher really just be a link instead of a vue component?

[mattstauffer]

Hitting pause for the night, but just in case anyone else takes a look, I moved preferences to be a JSON column on the user, and added a convenience method on the user named preferences().

It functions like the global config() helper; pass it a key and you get that key. Pass it a key and a value, you get that key and if it's not set you get the value. Pass it an associative array and it saves the contents of that array.

However, there's one big difference; it has defaults pre-set for each key, and you can only set keys that are in the pre-set list of preferences. So as we add a preference, we'll add its key, it's possible options, and its default into that list.

[mattstauffer]

I have decided to keep the resource switcher in Vue for now because it's the cleanest way to POST to the endpoint making the change.

[wotta]

I took the liberty to go through this pr because I noticed it had "a lot" of files.

Commented on some files with questions and suggestions. I also have a question about the public app.css and app.js.
Should those files be included to git? Reason is that it feels really weird to me to have "compiled" files in git.

I would also suggest to try to keep the pr a little bit smaller so it is more easy to go through the changes that were made.

[mattstauffer]

I would also suggest to try to keep the pr a little bit smaller so it is more easy to go through the changes that were made.

Yep.

[mattstauffer]

Allowing a few todo's in because they relate to work addressed in #115