Added post-checkout hook for submodule updates

[cmraible]

no issue

• Currently to work on Ghost locally, you have to use the --recurse-submodules flag when first checking it out with git clone .....
• Adding git submodule update --init --recursive to a post-checkout hook accomplishes the same thing, without the user having to know the exact correct command to run when cloning.
• This allows us to use a simpler git clone ... command.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 73.83%. Comparing base (607dee2) to head (853d0a8). Report is 318 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #21287      +/-   ##
==========================================
+ Coverage   73.70%   73.83%   +0.13%     
==========================================
  Files        1285     1284       -1     
  Lines       76010    76106      +96     
  Branches    10114    10130      +16     
==========================================
+ Hits        56021    56195     +174     
+ Misses      19060    18975      -85     
- Partials      929      936       +7     

Flag	Coverage Δ
admin-tests	46.57% <ø> (+0.47%)	:arrow_up:
e2e-tests	81.71% <ø> (+0.04%)	:arrow_up:

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[cmraible]

Realized we can do this in the prepare script (along with installing git hooks), which accomplishes the same thing essentially. This approach didn't seem to work as I expected it to — it worked when changing branches, but not immediately after a fresh clone.