[IMP] models: when checking for data in a table, do not use column 'id'

Replace the query "SELECT min(id) FROM xxx" by "SELECT 1 FROM xxx LIMIT 1".
Both requests are as efficient, and the second one does not crash if column
'id' is missing.
[FIX] website: prevent t-field on selection fields

This quick fix avoids a buggy behaviour in version 8.0 that could
confuse users.

A future version should implement properly selection fields in t-field.

(closes #2490)

(cherry picked from commit fe3cac30e4c5c132da1de02576d4aa325979ccd9)
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64