Improve translation flexibility

[Arkkimaagi]

As I have described in #876 the translation system could be improved with better flexibility in word reordering.

With this code, support for flexible translations can be added with a case by case approach. This change is backwards compatible and should not break existing translations. (As far as I know)

I have provided a sample translation within the default calendar module that solves Finnish language problem with the word order.

If this merge request is accepted, moving from code like this:

this.translate("RUNNING") + " " + moment(event.endDate, "x").fromNow(true)

to code like this

this.translate("RUNNING", {
  fallback: this.translate("RUNNING") + " {timeUntilEnd}",
  timeUntilEnd: moment(event.endDate, "x").fromNow(true)
})

Would allow translation like this:

"RUNNING": "Päättyy {timeUntilEnd} päästä",

to result:

Päättyy viiden minuutin päästä

while falling back to old behaviour with the fallback option given in the variables.

The new translator function checks for the variables object and if it exists, it tries to replace all the variables within the given translation template with ones found in the variables. If the translation template does not contain any variables in curly braces, it checks if there's an fallback option in the variables and uses that to generate the old style translation.

This also fixes the Finnish translation for RUNNING events. Instead of saying something like "Ongoing in 5 minutes" like the current translation says, it is fixed to "Ends in 5 minutes" in Finnish.

[MichMich]

Awesome work! It does need some documentation though. Could you update the Readme and Changelog? Thanks!

[Arkkimaagi]

I will make my suggestion for updating these files, but it will take a couple of days to do as I'm quite busy atm.

[Arkkimaagi]

Anything else missing? Please feel free to edit the documentation to your liking, I tried. ;)