Add Serbian Latin locale with transliteration of Serbian Cyrillic locale

[ocean90]

Background: ​https://make.wordpress.org/polyglots/2017/12/07/revisiting-serbian-latin/

Todos:

• Create a new locale sr_SR_latin
• Set up translation sets on translate.w.org for sr/latin
• Make translation sets for sr/latin read-only
• Add a task which automatically copies new translations from sr/default with transliteration to sr/latin
• tbd;

[ocean90]

In 10357:

Translate: Don't show suggestions if user cannot add/edit translations.

See #5471.

[ocean90]

In 10358:

Translate: Introduce the concept of read-only translation sets.

See #5471.

[ocean90]

In 10359:

Translate: Add locale definition for Serbian (Latin).

See #5471.

[ocean90]

In 10360:

Translate: First pass at adding transliteration support for Serbian locale.

See #5471.