[fix] concatenate contexts as strings

In some cases actual context string can't be transformed into the
dictionary using 'eval', for example if context string contains yet
undefined variable names (active_id, etc.). Because of it, added
workaround code and values to persist are now added to actual context using
string concatenation, without transforming actual context into the
dictionary (i.e. without using of eval).

 On branch 7.0
 Changes to be committed:

	modified:   addons/web/controllers/main.py