Added ability to change FullCalendar locale

[jaspwr]

Adds a setting allowing users to change FullCalendar locale and autodetects the users locale for the default setting.
Fixes #355.

[davish]

Thanks for the PR @jaspwr! One comment -- new settings must be added as optional/potentially undefined in the typescript, since old users updating won't have that setting defined. Making that change will likely cause typescript errors where that undefined isn't handled, so after fixing those, I'd be happy to merge this. It would be a great addition!

[jaspwr]

Should be fixed now.

[davish]

I just tested this locally, and I couldn't find en-us as a locale option, so I don't think the dropdown is displaying all possible options.

[jaspwr]

I had a look at that allLocales list and it seems to be generated from all of the items in this folder (see here) and as en-us (or just en as FullCalendar calls it) is the default and the files in this folder override its properties, it's not included in this list. This might be an oversight and should probably be fixed upstream if that is the case. In the live demo they provide they use getAvailableLocaleCodes to achieve this however that method is on the actual Calendar object and as far as I know there is no Calendar instance accessible in settings.ts. Of course there's the hacky solution of just appending "en" to the list but I'll have a better look at this later and try find the best solution. Otherwise I can just try get allLocales fixed upstream.

[jaspwr]

I changed it to add en to the list manually in 7be9a8c and gave it the label en-us because I think that is less confusing.

[gfpoliva]

Hey @davish, isn't it ready to merge?