Actions: Make mail-message-new uniform, add 16px

[newhoa]

mail-message-new symbolic did not match the 24px version (which was updated more recently in 367c69b). So updated the symbolic icon to match.

Also added a 16px version.

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I worked on a 16px mail-send-receive to match the symbolic version. Wasn't sure whether to include it in this PR. If it's wanted I can add it here or in another PR, just let me know.

Currently, no fullcolor version of this icon exists. I tried 3 options:

1. Orange and Green based on this older version, which elementary-xfce still uses.
2. Purple and Green based on document-send and browser-download
3. Purple and Blue based on import, export (although mail does already use these colors for forward and reply)

Anyway, let me know if you all want this one and if so what version and whether to include it in this PR or another.

Thanks!

[danirabbit]

Thanks for keeping these consistent!

And yeah totally interested in the color mail-send-receive! I think the purple and green one to be consistent with upload/download looks really good personally!