Create constants for the retry functionality

Currently, we define the number of retries and the initial back-off time at the top of each file as
```python
MAX_RETRY_ATTEMPTS = URLs.connect_max_retries
BACKOFF_INITIAL_DELAY = 5 # seconds
```
Both constants should be defined in the `constants.py` file and imported at the start of each file. This will also result in the same back-off for all cogs.
