Create system to invite speakers to the CfP

[SamLR]

create a user & empty proposal of a specific type for them, put it in manual review & email them a link to fill it out.

see #1327 & #813

Flow is

1. Fill out form:
   • name + email (both required)
   • title (otherwise defaults to [How should this appear in the schedule?]
   • description (default: [A more detailed description])
2. Creates a new user & proposals, adds them to the db
3. Sends the user a login link with next set to /cfp/proposal/<new id>/edit which is the account edit page

To make this work any proposal in 'manual review' can now be edited at any time (but this should be fine as we'll be paying more attention to these)

[SamLR]

@Jonty can you check the wording of the email?

There's probably a follow up task to make the email customisable but I wanted something out

[Jonty]

@SamLR I'd like to tweak this a bit, but I am tired. Tomorrow!

[SamLR]

@SamLR I'd like to tweak this a bit, but I am tired. Tomorrow!

Thought you would. I am also tired otherwise I'd have poked the flows & text a bit more but I think (hope) it's at polish phase now

[SamLR]

closed in favour of #1353