Mirador 2.4.0 - Search-within, annotation and build system improvements

• This release integrates a long-awaited IIIF feature to allow searching all the annotations on a manifest in one place if the manifest contains given a IIIF-compliant search backend.
• Search results can be clicked to navigate to the appropriate canvas for viewing them.
• Multiple hits on the same page a accessible by clicking comment icons, and the viewer with zoom and pan to their positions on the canvas.
• Search-within is considered a beta or test feature and is disabled by default. The configuration options can enable it for experimentation.
• Extensive additional testing on annotation retrieval has been added to support the addition of the search-within feature.
• HTML lists are now preserved in formatted annotation bodies.
• Errors arising from the inconsistent implementation of annotation retrieval between synchronous (localStorage) and asynchronous (remote server-backed) endpoints have been caught, preventing some failures.
• A configuration option allowing the addition of custom buttons to the window has been added.
• An event indicating the successful rendering of the main menu header has been added to the API, giving some indication that manipulation by enveloping scripts can safely commence on Mirador's initial generated DOM.
• A bug causing metadata to parse incorrectly if it was a simple string has been fixed.