Navigating left/right through {placeholders} in HTML property "values" is buggy in RTL messages

[eemeli]

This is a regression from #2926, which fixed the visual appearance of RTL messages, but made keyboard navigation through them trickier. For examples, see these strings:

• https://mozilla-pontoon-staging.herokuapp.com/he/firefox-accounts/all-resources/?string=237716
• https://mozilla-pontoon-staging.herokuapp.com/he/firefox-accounts/all-resources/?string=237717

When navigating with left/right arrow keys, the cursor may jump oddly around placeholders in properties.

The core problem here is that the values of HTML properties may be either LTR or RTL, so need to use dir="auto", which leads to some bugginess in our implementation. This is probably not a browser bug.