Unexpected shift + enter behaviour when updating grant's funders.

**Problem**

Typing `shift + enter` to send an update message breaks a deep-seated mental habit. In messaging apps, `shift + enter` is usually used when you want to create a new line / paragraph *without* sending. This habit is hard to modify, even if it's clearly stated that `shift + enter` will perform something different. As it stands, some users will press `shift + enter` to try to create a new line / paragraph before realising that they've actually sent an incomplete message (disclaimer: I am one of those users!).

**Proposed solution**

Bring the behaviour in line with a user's existing mental model: prevent `shift + enter` from sending the message (it should create a line break instead). Additionally, I'd recommend making the `Post` button slightly larger.