Allow signing PDFs with stored signatures

[Proteus]

This is similar to the Preview app in OSX. Firefox already allows annotations on demand to draw a signature on a PDF. The idea is to

1. Store the signature annotation and Sync it.

2. View stored signatures on devices and be able to select one to be used as a signature annotation.

Optionally one could protect the signatures using the master password mechanism used to protect stored logins.

[Proteus]

@marcoI was so excited about trying the new signature feature today and was tracking 137 rollout. If someone wants to try the feature now, set pdfjs.enableSignatureEditor to true in about:config

Signatures, however, do not sync across devices. Did this not make it in this release? Is it on the roadmap?

[marco]

Syncing didn't make it yet, it is on the roadmap!

[Jon]

Great news...

This feature is now being delivered — see @marco's recent comments for more info.

Thanks to all for your votes and comments that helped push this work forward 💪