1626186 - Add RTL support for RDM

I'm not sure if that's a good idea to add RTL support for RDM, but in case it does, the following changes are required:

Add dir="rtl" to the RDM's <html> if the UI is RTL
Convert almost all of the CSS properties to logical ones in devtools/client/responsive/index.css and devtools/client/responsive/responsive-browser.css
Set direction: ltr on viewport-dimension (dimensions should always be displayed as LTR), and align in to the start position in the custom device dialog
Set the inputs in the custom device dialog to unicode-bidi: plaintext with a text-align: match-parent. Ditto for .device-label > span
Mirror viewport-resize-handle for RTL and change its cursor property to nesw-resize. Make sure it appears on the left side of the tab
When the viewport is set to be aligned to the left (actually right), make sure it actually does aligned to the right (is removing [1] enough?)
Make sure resizers work after the above changes, with the side resizer being on the left
Add in the comments for the responsive.settingOnboarding.content and responsive.settingOnboarding.content strings that "left" should be translated as "right" for RTL locales, and notify the RTL localizers of this change

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