Bug 1061215 - [Messages][Refactoring] Improve handling of remaining chars counter. r=schung

Well, we should either use BEM or shouldn't use it at all, being BEM-ish a bit isn't effective :) So if  we decide to use BEM starting from this patch, then we shouldn't have top\bottom\left\right, we need more semantic blocks, as I said previously I'm providing one ```.messsage-subject-composer``` in the next patch. So IMO we need smth like this:
```
.message-subject-composer__mms-label
.message-content-composer__mms-label
.message-content-composer__message-counter

// and arguably generic element selector as well
.mms-label
```

Using selectors like ```top-mms-label``` doesn't give us readability and grouping BEM benefits, it just replaces our 2-level selectors with 1-level ones. That's it.

What do you think? Should we start using real BEM now? I'm open to it, just want to avoid half-solutions.

Just as side note, adopting web components, will embrace us to be more BEM - we'll have separate folder for component (aka Block) with own JS\CSS files and, that's more important, with isolated own shadow DOM (yeah, it looks like I'm starting to advocate them :) but Gecko\Dev Tools bugs and performance uncertainty doesn't allow me to do it in full)