fix: #1600

Form.py - Fix issue1602 Peewee BaseModel

This fixes issue described in 1602 and 1606 where the Peewee ORM changed their function for BaseModel to ModelBase in Peewee 3.0 and greater. This should work for Peewee versions less than 3.0 as well.

https://github.com/flask-admin/flask-admin/issues/1600

Confusing...

Use table name rather than model name to construct filter query in QueryAjaxModelLoader

Drop support for python 2.6

Fixed x-editable does not show completely in bootstrap's responsive table

Use select2 in editable columns

Change fields.py to be compatible with SQLAlchemy>=1.2

returns peewee sorted columns list by default

refs #1536 fixed pk validate in InlineModelFormList

Use LEFT OUTER JOIN instead of INNER JOIN when searching rows

Use in scaffold_*_columns methods

Add support for postgres CITEXT type

citext is a case-insensitive character string type.
See https://www.postgresql.org/docs/current/static/citext.html

rediscli: prevent form submit when execute command

allow to use inherited form CharField or TextField fields for searching