Cleanup of plugin-single.php and related code

[joostdevalk]

The code in the partials plugin-single.php and section-advanced.php is slightly unwieldy and not very DRY. This patch aims to fix that.

It also:

• No longer has the containing block for closed plugins' icons.
• Introduces a couple of template functions in class-template.php to retrieve messages so we can maintain these in one place.
• Cleans up the code style a bit.

[joostdevalk]

Cleanup patch

[obenland]

Thanks for the patch @joostdevalk. By any chance, is this output from a script of some sort?
Most of the alignment and whitespace changes in class-template.php are not conforming to existing code styling guidelines as I understand them.

Would you mind resubmitting a patch without them?

[joostdevalk]

Better version of the patch

[joostdevalk]

Done @obenland

[obenland]

In 6250:

Plugins: Don't show banner for closed plugins.

See #3310.

[obenland]

In 6251:

Plugins: Simplify template logic.

Moves most of the logic into template tags to make the single-plugin template it bit easier to understand.

Props joostdevalk.
Fixes #3310.

[obenland]

In 6252:

Plugins: Restore some formatting changes

See #3310.