Fix problem in server with URI encoding of titles in etags

Fixes #398
Force position:relative on tag pills

This avoids a problem with tag pills not working properly within table
cells.
Add automatic saving and warning on exit with unsaved changes

We re-use some of the existing syncer mechanism. It was already keeping
track of changes to tiddlers in the store when working with a tiddler
syncadaptor. Now it also tracks changes when there is no syncadaptor,
allowing us to provide a warning if there are unsaved changes.