Use Recurrent Complex & Default World Generator instead of Lockdown

[J0B10]

Why I hate Lockdown

So generating the world with lockdown is producing a few issues & inconveniences:

• gamemode setting is visible but ignored. You will always start in survival
• inability to generate flat maps for testing while lockdown is enabled (only in singleplayer)
• every world has the same seed, therefore no randomization at all (even for stuff like local ember amount and local starlight)
• accidental changes to game rules of the template world would affect generated worlds
• any changes to the template world are quite tedious
• (we don't have COVID-19 in game 😜)

Default World Generator

Default World Generator allows creating and enforcing presets.
We can enforce generation of a void map while allowing control of other settings:

We could even replace Initial Inventory with give commands or a custom command (created with crafttweaker) that is run on world creation.

Also this allows having a cool preset selection window with a preview of the map and a short description.
In the future we could even add multiple Pack modes (like the expert mode in FTB-youKnowWhat) if desired.

Recurrent Complex

Recurrent Complex can be used to create the spawn island.
It allows saving it as .rcst and .schematic and then just importing the structure at spawn.

This has the following benefits:

• multiplayer servers can use the /#gen command to create multiple spawn islands
• the structure can be easily modified and if desired distributed separately
• better control over spawning entities
• ability to fill chests with random loot if desired
• we could randomize the properties of the collector crystal for astral sorcery
• (if desired) ability to create further custom structures
  (maybe in the end or nether to make it more interesting The Nether is almost useless #110 )

Recurrent Complex unfortunately has a bug that may slow down world generation ( Ivorforce/RecurrentComplex#155 ) or unlikely crash the server.
This probably won't be an issue if only one structure is generated but we should make sure to test it.
According to Ivorforce/RecurrentComplex#467 & Divine-Journey-2/Divine-Journey-2#158 using the older version 1.4.8.1 could also help.

Further reading material on this issue (that seems common among modpacks, including AllTheMods2):
https://www.reddit.com/r/feedthebeast/comments/5x0twz/investigating_extreme_worldgen_lag/