* Properly determine when the popup encounters an error sending to a tab
  due to the tab either not being loaded, or being a forbidden page.
* Indicate in the sidebar when the page is forbidden (content scripts
  will not run).
* Indicate in the sidebar when the user has chosen to use the pop-up
  instead, and ask the user to close the sidebar.
* Remove some more console.log (the last, in fact) from src/

Note: several messages now need to be added regarding Landmarks being
      unable to run (applicable to all browsers) and sidebar-specific
      ones.

* Preliminary support for displaying an explanatory message about the
  interface choice.
* Simplify the CSS using better selectors.
* Add some more spacing for legibility.

Notes:
* Need to translate stuff!
* Need to figure out text alignment browser-agnostic-wise.

* Create panel.html and panel.js dynamically during the build, by
  copying popup.html and popup.js. The only difference in the HTML is that
  the sidebar version of the popup code is included in the sidebar
  panel.html (via text replacement during build). The code differences are
  around what the sidebar does when it starts up (and, in future, letting
  the user know that the popup will be used if the interface setting is
  set/changed that way).
* Clean up output of the build script somewhat.
* Move the ESLint configs closer to the code. In future, splash.js will
  probably be moved to src/code/ and src/static/.eslintrc.json would be
  removed.

* The sidebar will always update if shown, even if it is not the primary
  interface. This is becuase we cannot close it when it is disabled, there
  is no need to limit it to not working properly when disabled.
* Insert a (currently placeholder) warning about the interface settings
  when the sidebar is open but is not the primary interface.
* Simplify the background script accordingly; it no longer needs to
  track if we think we should or should not be using the sidebar.

* Include explainer for chosen sidebar settings in translation messages.
* Tweak ordering of stuff in options CSS.
* Add info message to explain to the user when the sidebar is open but
  is not the primary interface.
* The message can be dismissed.
* The state of the dismissal is recorded and can be reset via the
  options page.

Note:
* The messages are just placeholders and need translating.
* Eventually different messages will be needed for Opera (currently
  working on Firefox specifically).

* Add a button to open settings from the sidebar.
* Translate more messages.

On !{Firefox, Opera} the regex for replacing UI sections in options.html
did not allow for multiple separate UI bits. This was fixed by making it
both non-greedy and globally-matching.

* Add message for forbidden pages.
* Process some FIXMEs.
* "popup" -> "pop-up" in UI message.