[FIX] web: make server-wide modules ordered again

Rev. 279d928693 forced
`server_wide_modules` to include 'base' and 'web', because both contain
controllers that need to be always loaded.

However the patch used a set() that randomized the order of the list,
when combined with Python 3.5's randomized hash function.

This is turn could cause the checksum of asset bundles (web.assets_backend)
to randomly vary, which could cause rapid assets recycling and errors.

addons/web/controllers/main.py

@@ -178,11 +178,11 @@ def module_installed_bypass_session(dbname):
     return {}
 
 def module_boot(db=None):
-    server_wide_modules = {'base', 'web'} | set(odoo.conf.server_wide_modules)
-    serverside = []
+    server_wide_modules = odoo.conf.server_wide_modules or []
+    serverside = ['base', 'web']
     dbside = []
     for i in server_wide_modules:
-        if i in http.addons_manifest:
+        if i in http.addons_manifest and i not in serverside:
             serverside.append(i)
     monodb = db or db_monodb()
     if monodb: