annex: enable during installation

[kousu]

https://docs.gitea.io/en-us/git-lfs-setup/ says Git LFS is disabled by default

[server]
; Enables git-lfs support. true or false, default is false.
LFS_START_SERVER = true

however, in practice, it's usually turned on, because the install page fills in this text field

tracing

The reason this is filled in by default is because it's initialized from [lfs.PATH]

https://github.com/neuropoly/gitea/blob/fa43bce541507c8a702723f84764a48db9278506/routers/install/install.go#L122

and that is non-nil; according to https://docs.gitea.io/en-us/config-cheat-sheet/#lfs-lfs we have these defaults:

[lfs]
STORAGE_TYPE = local
PATH = ./data/lfs  # if STORAGE_TYPE == local

and then on submit, it's read back in by

https://github.com/neuropoly/gitea/blob/fa43bce541507c8a702723f84764a48db9278506/routers/install/install.go#L416-L418

which in turn turns on LFS_START_SERVER.

Anyway, this is an annoying inconsistency between LFS and annex, so add a checkbox to the install UI for git-annex, and leave it on by default to match the LFS behaviour.

[kousu]

(this didn't occur to me before because, for neurogitea, we're generating a app.ini ourselves and skipping the install UI entirely; so this isn't a high priority. but it would be good to polish this off)