fix(portal): show PO pay button if payments installed

[GursheenK]

Description
After payments app was made a soft dependency, the Pay button was removed from the Purchase Order page in the portal. Even though show_pay_button is enabled in Buying Settings, it doesn't actually show up even if payments app is installed.

Changes
Show the Pay button if payments app is installed and the show_pay_button setting is enabled.

Resolves #39599

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (31f586f) 60.12% compared to head (7475233) 60.13%.
Report is 5 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop   #39643   +/-   ##
========================================
  Coverage    60.12%   60.13%           
========================================
  Files          758      758           
  Lines        70951    70952    +1     
========================================
+ Hits         42662    42667    +5     
+ Misses       28289    28285    -4     

Files	Coverage Δ
...stock/doctype/material_request/material_request.py	69.76% <100.00%> (+0.08%)	⬆️
erpnext/templates/pages/order.py	0.00% <0.00%> (ø)

... and 4 files with indirect coverage changes

[s-aga-r]

changes reverted in PR.

[s-aga-r]

@GursheenK also disabled and hide the Show Pay Button field in Settings if the payments app is not installed.