Allow global opt-in to snippets

This is required for generic_lsp_completer thingies that _only_ support
snippets (like json, yaml) and allows the configuration to be global:

    {
      'name': 'json',
      'cmdline': [ 'node', s:lsp_dir . '/json/node_modules/.bin/vscode-json-languageserver', '--stdio' ],
      'filetypes': [ 'json' ],
      'capabilities': #{ textDocument: #{ completion: #{ completionItem: #{ snippetSupport: v:true } } } },
    },

We _also_ allow this to be set in extra conf files, but that's way less
convenient and we should probably ditch it.

ycmd/completers/language_server/generic_lsp_completer.py

@@ -22,15 +22,25 @@
 class GenericLSPCompleter( language_server_completer.LanguageServerCompleter ):
   def __init__( self, user_options, server_settings ):
     self._name = server_settings[ 'name' ]
-    self._supported_filetypes = server_settings[ 'filetypes' ]
-    self._project_root_files = server_settings.get( 'project_root_files', [] )
+    self._server_settings = server_settings
+
     super().__init__( user_options )
+
     self._command_line = server_settings[ 'cmdline' ]
     self._command_line[ 0 ] = utils.FindExecutable( self._command_line[ 0 ] )
 
 
+  def GetSettings( self, module, request_data ):
+    settings = {}
+    settings = super().GetSettings( module, request_data )
+    utils.UpdateDict( settings.setdefault( 'capabilities', {} ),
+                      self._server_settings.get( 'capabilities', {} ) )
+    return settings
+
+
+
   def GetProjectRootFiles( self ):
-    return self._project_root_files
+    return self._server_settings.get( 'project_root_files', [] )
 
 
   def Language( self ):
@@ -76,4 +86,4 @@ def GetCodepointForCompletionRequest( self, request_data ):
 
 
   def SupportedFiletypes( self ):
-    return self._supported_filetypes
+    return self._server_settings[ 'filetypes' ]