[bukuserver] Don't allow UPPER CASE tags

[GreenLunar]

Bookmark in question:

1. Free Software Foundation [1]
   > https://www.fsf.org/
   + The FSF is a charity with a worldwide mission to advance software freedom.
   # brand:GNU, foundation:Free-Software, Free-Software

Original set of tags:

brand:GNU, foundation:Free-Software, Free-Software

After import:

brand:gnu, foundation:free-software, free-software

After editing inside bukuserver web ui:

brand:GNU, foundation:Free-Software, Free-Software

Consequential problem

• bukuserver isn't able to locate that specific bookmark by searching for tags "foundation:Free-Software" & "Free-Software" nor "foundation:free-software" & "free-software".
• bukuserver IS able to locate the bookmark in question by searching for tag "brand:GNU", but not "brand:gnu".

P.S. buku (command-line) handles all of the above queries well.

[GreenLunar]

Confirmed. Closing.