removed computedproperties in favor of properties

There are several computed properties in main/modelx.py BaseX like
- ```created_ago```
- ```modified_ago```
- ```created_utc```
- ```modified_utc```

Computed properties ([1]) are both computed whenever read and saved in
the datastore for the purpose of indexing. The idea is that you can still
query on the field. This causes at least 3 additional writes per object
being written (1 for the data itself, 2 for index asc and desc).

Aside from causing additional writes there seems to be a problem: all
four are depending on self.modified and self.created which are
```DateTimeProperties``` ([2]). The values of ```auto_now``` and
```auto_now_add``` are only set when the model is written.
I experienced non deterministic ```None``` values of these in the
Datastore.

I see two other problems here:

1. ```created_ago``` and ```modified_ago``` just should never be saved
in the datastore, as ordering by these two just doesn't make any sense.
2. ```created_utc``` and ```modified_utc``` imply that the normal times
are saved in localtime. They're not, appengine uses UTC.

I suggest transforming all these into ordinary properties.

[1]: https://developers.google.com/appengine/docs/python/ndb/properties#computed
[2]: https://developers.google.com/appengine/docs/python/ndb/properties#Date_and_Time
also change avatar_url ComputedProperty to property
Merge pull request #1 from joernhees/fix-computedprops

Change ComputedProperty to property in BaseX