Support Plurr preview for simple {FOO}-style placeholders

[iafan]

Right now we look for {FOO:<...>} style placeholders in the string to determine whether we need to show the live Plurr preview panel. However, we could expand that to any strings that contain simple placeholders like {FOO} as well, since these are also valid in Plurr (and Plurr can be used to render such strings even if they don't contain plurals or choices).

Example of such a string:

[benjamimo1]

Since this ticket is similar to the Emoji Highlight ticket I'll take a look
Relevant: https://github.com/loctools/plurr

Plurr: Cross-platform format specification + libraries for handling plurals/genders/conditionals for Go, Java, JS, Obj-C, Perl, PHP, Python

[benjamimo1]

@iafan Do you have an example (unit) of a live Plurr preview panel?
EDIT non zing example:

http://iafan.github.io/plurr-demo/#locale=en&autoPlurals=true&strict=true&s=Do+you+want+to+delete+%7BN_PLURAL%3Athis+%7BN%7D+file%7Cthese+%7BN%7D+files%7D+permanently%3F&p%5BN%5D=5

[iafan]

@benjamimo1 here you go: https://translate.evernote.com/ru/android_evernote/translate/strings.xml.po#unit=6140694

You can find such units in Zing by searching for _PLURAL.

[benjamimo1]

Relevant paths:

/pootle/static/js/editor/components/plurr/PlurrPreview.js
/pootle/static/js/editor/components/plurr/PlurrEditor.js

[benjamimo1]

@iafan can you reference a code line where "Right now we look for {FOO:<...>} " happens?

[iafan]

@benjamimo1 I'd start here: https://github.com/evernote/zing/blob/2d130c029affe82a5918e539fcbc1913bf8c9b7f/pootle/static/js/editor/containers/EditorContainer.js#L73

[benjamimo1]

Current status tested both with one and multiple placeholders: