BibMerge: add subfield sorting and interface fixes

• Sorts subfields in merge preview

• Adds "Compare" button

• Disables the "submit" button if there are no records loaded yet or there is an error in the url

• Makes second record ID mandatory (no more empty record2 errors)

• Stops freezing the record ID input fields and "Compare" button when given ID doesn't exists

• Adds parsing of the records IDs (fixes throwing errors when there were invalid characters in record ID)