Add templating functions for working with URLs

[StopMotionCuber]

This is adding 2 functions to templating to improve ability to work with URLs in templates.

These functions are

• safeUrl (the same as safeHtml, but for URLs)
• urlUnescape (transforming stuff like %20 to blanks)

For relevant context on why I need that, see #4616. I'm also fine with dropping either of the functions for my issue. safeUrl might have some security implications, while urlUnescape can already be implemented quite inefficiently with a long chain of reReplaceAll and therefore should have less security implications

[Spaceman1701]

I see no problem with this

• I don't think adding safeUrl introduces any security concerns that safeHtml doesn't already
• urlUnescape seems useful

There's certainly a benefit to keeping the number of exposed functions lower, but these functions meet the bar of being generically useful and hard to replicate with what's already provided.

We'll have to wait for a real maintainer to approve before it can be merged.

[StopMotionCuber]

What's missing for this to be merged?

[StopMotionCuber]

I kinda fatfingered closing this PR (some weird keyboard shortcuts while trying to type).
Well, to repeat the question above, what's missing here to get this merged? Kinda bummer that this didn't make it into the last release, especially as the alertmanager release cycle is rather slow