Refine behavior for adding new lines in messages

[mattermod]

This applies applies to both Mac & Windows

When Advanced > send messages on CTRL-ENTER is set to "Off", allow the following keys to insert new lines:

1. Shift-ENTER (Already implemented)
2. CTRL-ENTER
3. ALT-ENTER (Already implemented)

______

When Advanced > send messages on CTRL-ENTER is set to "On for all messages", allow the following keys to insert new lines:

1. ENTER (Already implemented)
2. Shift-ENTER (Already implemented)
3. ALT-ENTER (Already implemented)

______

When Advanced > send messages on CTRL-ENTER is set to "On only for code blocks starting with ```",

1. Allow the following keys to insert new lines inside a code block:

1. ENTER (Already implemented)
2. Shift-ENTER (Already implemented)
3. ALT-ENTER (Already implemented)

2. Allow the following keys to insert new lines when not in a code block

1. Shift-ENTER (Already implemented)
2. CTRL-ENTER
3. ALT-ENTER (Already implemented)

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JIRA: https://mattermost.atlassian.net/browse/MM-9343

[CSBatchelor]

I can do this one.

[hanzei]

Thanks for working on this @CSBatchelor 👍

[jasonblais]

Hey @CSBatchelor how are things going? Any questions so far? :)

[CSBatchelor]

Hi @jasonblais I haven't started on this yet, I just started a new full-time Software Engineer job and I forgot! I still want to do this though, I should be able to start on it within the next few days.

[jasonblais]

Great, thanks @CSBatchelor, and congrats on your new job! 🎉

[CSBatchelor]

Sorry I never got around to this, I unassigned myself so this issue can be picked up by someone else. I hope to work with Mattermost again in the near future!

[hanzei]

Thanks for the heads up @CSBatchelor 👍

[M-ZubairAhmed]

let me work on this, this is backend of frontend ticket as @deanwhillier puts it :P

[hanzei]

Awesome @M-ZubairAhmed, looking forward to your PR!

[esethna]

@M-ZubairAhmed how's this going? :)

[cinlloc]

Hi, I would like to work on this ✋

[cinlloc]

Hi, after having read the code it seems a bit complex for me at this time, I give up for now.

[mmospanenko]

[Boards] Jump over words (cmd+left/right) do not work too. Very annoying thing)