feat(examples): add r/morgan/{home,guestbook}

[thehowl]

I wanted a simple realm that allows any user to interact with it and see something happen. This is a guestbook realm; it allows anyone to leave a small message, which will be associated with their address.

It also is a nice showcase of how to effectively do pagination with AVL trees. I don't think we have many such examples so far.

[thehowl]

Leon pointed out that we have r/demo/userbook, which achieves something similar. We can consider whether to keep both, or just one or the other.

I see two advantages with this realm: using avl.Trees + pagination, and providing users with a way to actually "write something" (with the consequence of moderation -- hence why there is an admin function the remove a user's post; though obviously with the faucet working anyone can just have another account).

cc @moul

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.45%. Comparing base (68b8f54) to head (c76bba1).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2345      +/-   ##
==========================================
+ Coverage   60.44%   60.45%   +0.01%     
==========================================
  Files         563      563              
  Lines       75157    75159       +2     
==========================================
+ Hits        45426    45436      +10     
+ Misses      26341    26333       -8     
  Partials     3390     3390              

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contribs/gnodev	61.46% <ø> (ø)
contribs/gnofaucet	14.46% <ø> (-0.86%)	⬇️
gno.land	67.21% <ø> (ø)
gnovm	64.46% <100.00%> (+<0.01%)	⬆️
misc/genstd	80.54% <ø> (ø)
misc/logos	20.23% <ø> (ø)
tm2	62.10% <ø> (+0.12%)	⬆️

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[moul]

Looks good, but we should consider renaming this example to avoid conflicts with other guestbook implementations.

[thehowl]

Moved it to r/morgan/guestbook and added a small home realm. It seems appropriate for now.