On-page help for users who want guidance in writing

[MyfanwyNixon]

Out of #339. See also https://blogs.mysociety.org/internal/2016/09/19/quick-thought/

Ideas so far:

• (from @octopusinvitro ) A toggle on/off template within the compose screen. This could, eg, offer greyed prompts ordered appropriately, such as 'why are you writing? 'What do you want your [MP] to do?'
• (from @abimysoc) On-page pointers such as we have for FixMyStreet ("be concise", "tell your [MP] what you would like them to do")
• (From @jacksonj04) "how much computational overhead would it be to occasionally run a message through the rejection ruleset as people are typing, eg if they stop for a second or two we can slam it through a subset of the filters for duplicates, swearing, etc and throw up warnings if we think what they’ve currently written will get splatted?" <- I can see that getting quite annoying though :)

@dracos: also, please can you comment on whether we have any such functionality already? Abi says she had a feeling that we can detect all caps writing, but it didn't seem to when I tried it.