#96 Update proposal pages

[jessabean]

This PR addresses card 96 - Updating speaker proposal pages (show/edit/new) with a consistent layout.

Changes introduced:

• Moved event info bar to sit just below main nav
• Moved preview panel to a tab
• Added legends/headings to all fieldsets
• Moved tags field to just below abstract
• Updated comment styles
• Removed reviewer activity (no action to be taken, no particularly useful info being shown to user)

This PR does not address:

• tooltips
• label styles
• My Proposals layout

Screenshots:

@mghaught

[mghaught]

👍