admin: fix add value button of COLUMN_VECTOR does not work

It raised the Uncaught TypeError:
  "Object #<HTMLSpanElement> has no method 'add_record_inputbox'"
admin: improve error handling of select

Now it shows error message and hides loading dialog.
admin: submit search form with the enter key

It needed to click the submit button.
admin: show placeholder in search form

Usage examples would be useful because typing only keywords,
like Google search, results in an error.
Merge pull request #103 from orangain/feature-web-admin

Small improvements to the admin web interface

Patch by orangain. Thanks!!!