prep work for manifest V3 - move pgp functionality from background script to content script

[tomholub]

First should do some prep work - move what we can from background script to content script

Originally posted by @tomholub in #1509 (comment)

[tomholub]

For example:

• decrypting, and all OpenPGP.js operations

Following cannot be done yet:

• network requests (have problems from content scripts)
• storage related requests

[tomholub]

Background:
https://developer.chrome.com/extensions/migrating_to_manifest_v3

There will be no more background pages. Our background page mostly responds to BrowserMsgs from content script frames.

To the extent we can, we'll try to start moving functionality from background page to content scripts. This means that content scripts should load OpengPGP.js library (if they don't already) and as a part of this issue, individual iframes (such as pgp_block) will be calling their parent frame (which is Gmail content script) and not the background page. Specifically these calls:

Will now be listeners in main email provider content script instead.

[limonte]

TODO:

• pgpMsgType (Move pgpMsgType from background script #2552)
• pgpMsgDiagnosePubkeys (Move diagnosePubkeys() from background script to content script #2507)
• pgpHashChallengeAnswer (Move pgpHashChallengeAnswer from background script #2553)
• pgpMsgDecrypt (issue #2158 pgp back to bg script #2554)
• pgpMsgVerifyDetached (issue #2158 pgp back to bg script #2554)
• pgpKeyDetails (issue #2158 pgp back to bg script #2554)