Support missing po files [Originates from frontend-experiments branch]

[jatindhankhar]

This enables build process to continue in case of po files are missing.
This is done in two parts
First we add fallback values in the model itself

We change MANDATORY_ATTRIBUTES and remove categories and summary and retain following

MANDATORY_ATTRIBUTES = ['name', 'bundle_id', 'license',
                                'icon', 'activity_version']

License is necessary and recent analysis shows that very few repos are without license and to avoid any legal problems, aslo should clearly depict license of the activity (whatever is decided by the author)

summary = me.DictField(required=True,default={'en' : "Unknown Summary"})
categories = me.ListField(me.StringField(max_length=30),default=["unknown"])

Then we add default values for i18n in case translations are missing

 if any(translations):
       metadata['i18n_name'] = i18n.translate_field(metadata['name'],
                                                 translations)
       metadata['i18n_summary'] = i18n.translate_field(metadata['summary'],
                                                    translations)
    else:
        metadata['i18n_name'] = {'en' : metadata['name']}
        if 'summary' not in metadata:
            metadata['i18n_summary'] = {'en' : "No summary"}
        else:
             metadata['i18n_summary'] = {'en' : metadata['summary']} 

[scanterog]

Added some minor fixes and merged into master.