Get rid of 'templateComposerContextPath' in channel manager as it is not needed

The channel manager has quite some 'templateComposerContextPath' and 'contextPath' logic. The first one was meant as the context path to fetch the channels over, the second to actually render the channel. However, it seems that its usage in the channel manager has become quite random. Also, with the introduction of the _cmsrest calls that fetch all available channels over 127.0.0.1, the templateComposerContextPath has become pointless and confusing. The logic we should implement now is as follows:

1) The cms rest proxy over 127.0.0.1 needs to be configured with a contextpath to a webapp that is a hst app
2) The contextpath over which a channel should be retrieved in the channel mngr should be taken from the 'contextpath' of the channel