web: Extension UI improvements

[WumboSpasm]

I always thought the extension UI could use a bit of work, so this is my attempt at giving it a nice visual refresh.

General changes:

• Buttons, text/number fields, and dropdowns have been given a consistent style that reflects Ruffle's color scheme
• English text strings have been updated to make better use of given space

Pop-up menu changes:

• Fine-tuned padding
• Decreased font size by 5%
• Moved version text to underneath the logo
• Improved aesthetics of status indicator
• Replaced settings button into an icon

Settings page changes:

• Version text has been added underneath the logo
• Increased space between text and buttons
• Slightly decreased gap between options

Player changes:

• Improved spacing of elements in header bar
• Removed margins around Flash content
• Turned info modal into a toggleable sidebar

Some things to note:

• The settings icon in the pop-up menu appears slightly smaller in Firefox, despite looking fine when viewing the HTML directly
• The up/down arrows of the number input field are impossible to style in a way that's consistent across browsers, so they have been left untouched for now

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[adrian17]

LGTM (note: let's squash on merge)

[Dinnerbone]

Thank you!

The localisation changes are a bit unfortunate but it's for the best