Improve the handling of the materialized views

[mschoema]

Creating and removing materialized views has to be done carefully.

Below are different use-cases that need to be handled:

• Multiple QGIS projects from the same user on the same database
• Mulitple QGIS projects from different users on the same database

Currently, both cases work if all the projects working on the same database have different names.
This is not ideal, as we cannot require different users to know each other's project names.

Potential ideas:

• Use a unique name per user and combine it with the project name.
• Use a unique id per project instead of the project name.