Backport #27231 #27259 manually

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Co-authored-by: delvh <dev.lh@web.de>

I think it's better if the primary actions have primary color instead of
green which fits better into the overall single-color UI design. This PR
currently replaces every green button with primary:

<img width="141" alt="Screenshot 2023-09-16 at 14 07 59"
src="https://github.com/go-gitea/gitea/assets/115237/843c1e50-4fb2-4ec6-84ba-0efb9472dcbe">
<img width="161" alt="Screenshot 2023-09-16 at 14 07 51"
src="https://github.com/go-gitea/gitea/assets/115237/9442195a-a3b2-4a42-b262-8377d6f5c0d1">

Modal actions now use uncolored/primary instead of previous green/red
colors. I also removed the box-shadow on all basic buttons:

<img width="259" alt="Screenshot 2023-09-16 at 14 16 39"
src="https://github.com/go-gitea/gitea/assets/115237/5beea529-127a-44b0-8d4c-afa7b034a490">
<img width="261" alt="Screenshot 2023-09-16 at 14 17 42"
src="https://github.com/go-gitea/gitea/assets/115237/4757f7b2-4d46-49bc-a797-38bb28437b88">

The change currently includes the "Merge PR" button, for which we might
want to make an exception to match the icon color there:

<img width="442" alt="Screenshot 2023-09-16 at 14 33 53"
src="https://github.com/go-gitea/gitea/assets/115237/993ac1a5-c94d-4895-b76c-0d872181a70b">

* Add main aria landmark to templates
 * Adjust some titles to improve understanding of location in navigation

Contributed by @Forgejo

* Refactor `i18n` to `locale`

- Currently we're using the `i18n` variable naming for the `locale`
struct. This contains locale's specific information and cannot be used
for general i18n purpose, therefore refactoring it to `locale` makes
more sense.
- Ref: https://github.com/go-gitea/gitea/pull/20096#discussion_r906699200

* Update routers/install/install.go

* Add class to page content to unify top margin

Previously pages would individually set this margin but some didn't so
content would stick to the header without any space. Resolve this by
adding a new class that is added on all pages. The only place where we
remove this margin again is on the pages with menu or wrapper in the
header.

* fix admin notices

* fix team pages

* fix loading segment on gitgraph for arc-green

* fix last missing case

Co-authored-by: techknowlogick <techknowlogick@gitea.io>

* Autofocus on 2fa passcode field

* Autofocus on 2fa scratch code field

* Initial commit for 2FA support

Signed-off-by: Andrew <write@imaginarycode.com>

* Add vendored files

* Add missing depends

* A few clean ups

* Added improvements, proper encryption

* Better encryption key

* Simplify "key" generation

* Make 2FA enrollment page more robust

* Fix typo

* Rename twofa/2FA to TwoFactor

* UNIQUE INDEX -> UNIQUE