Make themes more flexible

[southerntofu]

• Are you doing the PR on the next branch?
• Have you created/updated the relevant documentation page(s)?
• The change is fully backwards-compatible

Fix for both #324 and #361. I open this PR to bring more attention so everyone can test and see if it makes them happy.

Following remarks here, i made sure this patch was fully backwards-compatible by copying the template's theme (inside tera) to "THEME/templates/index.html" in all cases, where THEME is the theme name.

In addition, I propose to copy all theme templates to "theme/templates/" where "theme" is a literal. This would allow site templates to extend the theme's template regardless of the theme name, so that you can swap themes seemlessly as long as they implement the same blocks in the base template. Would this be ok?

Waiting for feedback before i edit the docs

[southerntofu]

done. the section is formatted properly and tests pass

[Keats]

It looks like it works while being much simple?! Can you add tests for shortcodes and an actual include?

[Keats]

ping @southerntofu

[Keats]

I didn't mean to close all the PRs by deleting the branch :(

[southerntofu]

Can you add tests for shortcodes and an actual include?

done. i also updated the docs

In addition, I propose to copy all theme templates to "theme/templates/" where "theme" is a literal. This would allow site templates to extend the theme's template regardless of the theme name, so that you can swap themes seemlessly as long as they implement the same blocks in the base template. Would this be ok?

^ Would this change be welcome?

[Keats]

This would allow site templates to extend the theme's template regardless of the theme name, so that you can swap themes seemlessly as long as they implement the same blocks in the base template.

I don't think it's common enough. And right now you can easily tests multiple themes and pick the one you prefer, which was the original intent.

[Keats]

It looks good to me; anything else you want to add to the PR?