How can I make it so players can't destroy farmland by jumping on it?

If you don't mind using a server, the WorldGuard plugin can be used. To create a server with plugin support, this is a good tutorial:

Stop the server and delete "world", "world_nether" and "world_the_end". Copy the world to "world".

Download WorldGuard and add it into the server's "plugins" folder.

Start the server back up.

Set gamemode to adventure.

Give yourself cheats (/op <player>).

For a guide on how to use WorldGuard here's another tutorial (I just can't be bothered to write the whole thing):

The flag used to prevent block-trampling is "block-trampling" (not too surprisingly). It does not affect operators.

Unfortunately, to distribute the map you need to distribute the whole server folder (excluding "cache", for copyright reasons).

Sorry if this wasn't the most ideal solution.