Contact Method should be its own DB field, not prepended to submitted messages

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

We currently concatenated the contact method and message content before storing it in the database:

hushline/hushline/routes.py

Lines 214 to 218 in 53c65ee

	content = form.content.data
	contact_method = form.contact_method.data.strip() if form.contact_method.data else ""
	full_content = (
	f"Contact Method: {contact_method}\n\n{content}" if contact_method else content
	)

Describe the solution you'd like

This should be a separate field so that we can better display them to the recipient. This will also later allow for better i18n.

[brassy-endomorph]

Relates to #514

[glenn-sorrentino]

Downside of this is that we (currently) only PGP encrypt messages.