Feature: Allow users to customize fTelnet settings in ecWeb

MR !225 (merged) will now resize the fTelnet client to 80x35 when starting 'msweeper', to allow a full level 5.0 game to be played.

Rob pointed out users may want a screen larger than 80x25 for other reasons too, so I'd like to add a way for them to do this, and would like feedback on what you think would work best before proceeding @echicken @rswindell

A few potential options (but open to other suggestions):

• A dropdown in the sidebar with options for 80x25, 80x35, 80x50, and Custom (which would prompt the user for WxH) that would only appear if fTelnet is embedded on the current page (so not on the Forum for example)
• Display the dropdown in the navbar instead of the sidebar (ie next to the Dark switch)
• Add a Telnet Settings button next to the Connect via Telnet button, which would pop up a modal to display the screen size dropdown.
  • This could also house additional settings, for example letting them choose a slower bitrate