Add ability to make a "read only embed"

[cassidoo]

There's a request from Kyle Johnson to be able to make a "read-only embed" of PocketCal.

This would involve:

• Adding a button to the sidebar called "Embed"
• Making a popup modal to give the user the code (an iframe?) for the embed
• Adding some kind of ability to make the calendar not editable when inside an embed
• Maybe worth adding an "Edit on PocketCal" badge at the bottom of the embed for folks to click through