Style guide for Message Strings in PO files

[mattias-p]

The Zonemaster components contain a fair amount of text strings that are presented to end users. These strings use parentheses, single and double quotes and all-caps words in apparently inconsistent ways. We should have a writing style guideline for end-user facing text that spell out when to use which.

[mattias-p]

Each language should have its own style guide.