[Enhancement] Migrate from ECR to another templating language

[syeopite]

Is your enhancement request related to a problem? Please describe.

ECR files are loaded at compile time. This causes two problems:

1. A lengthily compilation time
2. A requirement to recompile Invidious each time the templates are updated contributing to a bad developer experience.

Describe the solution you'd like

Invidious should migrate to a runtime based templating engine such as https://github.com/straight-shoota/crinja

Describe alternatives you've considered

Keep it as-is

Additional context

[SamantazFox]

I wasn't aware that straight-shoota ported Jinja templates to Crystal, that's great :D
While that's low priority, moving to Jinja templates might be a good step towards community-made themes! (See #4113 (comment))

[ChunkyProgrammer]

Is there still a way to validate that the templates don't have runtime errors at compile time? I imagine not, but I'm asking because these errors likely wouldnt be caught by the CI workflow