Increase thumbnail resolution when using a SQL db to store thumbnails

[bulam]

When using a SQL db to store post thumbnail images in base64 format, the field is only limited to 65K characters, which limits the thumbnail resolution to 125px X 125px here, which is too low for modern screens.

Similar to this PR, we need to upgrade the field to MediumText here
and then increase the resolution here

Note, after updating to MediumText, a db table for an existing knowledge repo instance will have to be migrated to use MediumText as well

[naoyak]

See also #432