Bug 840038 - [MMS][User Story] Image rescaling to 300kB(Apply to composer attachment) - r=gnarf

Adds resizing step in Composer
Adds a resizing notification
Adds Composer.isResizing
Only show file too large error when not an image
Refactors getting mmsSizeLimitation
Defaults to 300k for mmsSizeLimitation
Adds "resize-image" locale string
Adds number: 1 to pick action

Closes gh-9854