Refactor quote anchoring logic to use async/await instead of Promise
chains to make control flow easier to follow and change.

 - Convert `getPageOffset` and `findPage` helper functions to use
   async/await rather than Promise chains.

 - Combine the `prioritizePages` and `findInPages` functions into a
   single `anchorQuote` function.

   Combining the two functions allows some redundant work to be
   eliminated. `findInPages` used to call `getPageOffset` for the
   current page on each iteration which would in turn fetch the text of
   every page until it reached the given offset. `prioritizePages` would
   do a very similar loop in order to enumerate pages close to the given
   offset.

 - Replace use of `TextQuoteAnchor.fromSelector` with direct use of
   `matchQuote`. `fromSelector` wasn't providing any value over just
   calling `matchQuote` directly, and it required a dummy DOM element to
   be created and have its text populated.