[WIP] Create members mail form

[Tarasa24]

Relates to: #522

This is still very much WIP...

Implemented / figured out so far:

• oAuth2 PKCE client side auth token obtaining
• basic idea of the flow member would go through
• basic population of the sending form with user data (fullname, email, subscriptions = which shows can member mail)

Nearterm Todo:

• Obtain proper client id
• Pair the current logic to memberful API

More high level todo:

• Decide on message procesing solution
• Decide on rate limits

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Screenshots for your convenience:

Unauthenticated:

Authenticated:

Flow: