Treat web json properties as appendables

[jamoore]

In order to prevent hard-coding values in OMERO.webapp setup, omeroweb should append values rather than overwriting:

• ​https://github.com/openmicroscopy/openmicroscopy/pull/1905 will add append/remove functions which treat config.xml strings as json lists
• settings.py will read the json lists but not overwrite the value
• Each append value from the json list will be appened IFF it does not already exist in the default. (This might should be configurable)

A current limitation with this method for discussion is that it would not be possible to remove from the default, only to add to it.

NB: this is the first step toward a larger re-configuration effort that is needed. Including:

• bin/omero configjs or similar.
• Self-registering plugins

[jamoore]

For a slightly similar implementation, more complete but a bit more complex, see Andreas' ​https://github.com/knabar/openmicroscopy/commit/785fee6cb7b2cf6e667a953b966f4eb7b83291b8

[jamoore]

PR opened: ​https://github.com/openmicroscopy/openmicroscopy/pull/2028

[aknab]

Different implementation based on ​https://github.com/openmicroscopy/openmicroscopy/pull/1905 at ​https://github.com/openmicroscopy/openmicroscopy/pull/2030.

• Allows adding to and removing from the default value
• Allows multiple independent operations on the same settings, e.g. when installing multiple OMERO.web plugins

[sbesson]

PR ​https://github.com/openmicroscopy/openmicroscopy/pull/2041 opened against dev_5_0 for 5.0.0-rc2