Drag'n'Drop support

[StCyr]

Drag'n'Drop support implementation with https://github.com/Vivify-Ideas/vue-draggable

Remaining issues:

• It seems the code isn't properly initialized: If you load the mail apps from your browser, the drag'n'drop functionality won't work directly. You first have to perform an action (eg: selecting another email than the default one displayed) before the drag'n'drop functionality will work;
• Shouldn't navigate when the moved message is not tghe one currently displayed
• Do not expand when folder's account is not the same as envelope account
• Polishing

This PR is a continuation of #1753

[StCyr]

Hello @jancborchardt

I don't know if D&D support is the best option we have to classify e-mails.

I was also thinking about adding a "move" action in the same way how it is done in the "file" app:

[jancborchardt]

@StCyr fully agree → I moved my feedback into the issue which already exists about that: #255 (comment) :)

[StCyr]

This is very old and the move feature introduced in #3591 will solve the issue this PR tried to solve