Bench's nginx config should be fully user editable

[Xieyt]

Description User should be able to edit the config file ~/frappe/sites/<sitename>/config/nginx/conf.d/default.conf.

Expected Behavior

• User edits the Nginx config file and it's not altered by FM in any way.
• At the time of upstream config changes from FM(during migration):
  • The previous Nginx config file should be moved as a backup file.
  • The user has to manually move their previous custom config changes to the new config file.
• If default.conf doesn't exits then create it.

  [!CAUTION] If user removes custom folder include from the default.conf then admin-tools will stop working.

      # include custom config
       include /etc/nginx/custom/*.conf;

Current Behavior (if applicable)

• Currently user is only able to override some of the directives defined in default.conf or add new directives if they want using the custom directory.
• Changes done in default.conf doesn't persist.
• Changes are removed when container's entrypoint is ran.