[Commands] Add #unmemspell and #unmemspells Commands. (#1867)

- Add #unmemspell [Spell ID] command to unmemorize a spell by ID from you or your target.
- Add #unmemspells command to unmemorize all spells from you or your target.
- Cleanup #memspell command and change arguments from #memspell [Slot] [Spell ID] to #memspell [Spell ID] [Spell Gem] for easier use.
- Add #memspell [Spell ID] functionality to memorize to first open spell gem if there are any using FindEmptyMemSlot helper method.
- Rename client->FindMemmedSpellByID(spell_id) to FindMemmedSpellBySpellID(spell_id).
- Add client->FindEmptyMemSlot() helper method.
- Add $client->FindEmptyMemSlot() to Perl.
- Add client:FindEmptyMemSlot() to Lua.
- Add $client->FindMemmedSpellBySpellID(spell_id) to Perl.
- Add client:FindMemmedSpellBySpellID(spell_id) to Lua.

zone/CMakeLists.txt

@@ -541,6 +541,8 @@ SET(gm_commands
     gm_commands/undyeme.cpp
     gm_commands/unfreeze.cpp
     gm_commands/unlock.cpp
+    gm_commands/unmemspell.cpp
+    gm_commands/unmemspells.cpp
     gm_commands/unscribespell.cpp
     gm_commands/unscribespells.cpp
     gm_commands/untraindisc.cpp