add Sprig functions to go templates

[wunter8]

fixes #1121

Sprig is the library used on https://repeatit.io

[Devwanabe]

Missing connection to the server through my gateway connection. Am I using incorrect source code? What am I doing wrong

[binwiederhier]

Hm. This adds 9 dependencies to ntfy for such a tiny little mini feature. 9 dependencies are 9 chances to be injected with some random code from strangers and 9 chances for things to break in the future.

Maybe we can copy a subset of it, or just write some really useful ones ourselves? It's certainly cool to have these functions in there but damn 9 dependencies is brutal.

[wunter8]

Yeah, we can probably implement our own. It looks like there are about 70 functions total. Some of them we probably don't need (like the path functions), and some of them we probably don't even want (like the os env function)

Let me know which ones you think we should include/implement by hand