[#497] GWT code cleanup

Spreadsheet processing code related to spreadsheets
containing access point information
[#497] GWT code cleanup

Cleanup AccessPointDto and related code
[#497] GWT code cleanup

Cleanup AccessPointSummaryDto and related servlet classes
Merge pull request #540 from akvo/feature/497

[#497] GWT code cleanup
[#320] NO_IMEI should be treated as null

On WiFi only mobile devices (no SIM, no imei) the dashboard was storing
"NO_IMEI" as value, and that makes all this devices to be handled as the
same.

If we try to get a device by `imei` and is "NO_IMEI" we immediatly
return `null`, and the related code needs to search by `phoneNumber`.

`phoneNumber` property stores either the actual mobile phone number or
the MAC address of the device.
[#320] Make private constant
[#320] Move constant to Device domain class, adapt related code

Move the `NO_IMEI` constant to Device domain class, and adapt related
code (the classes calling `DeviceDAO.getByImei`
[#541] Requeue task for failed URL fetch
Merge pull request #542 from akvo/issue/541

[#541] Requeue task for failed URL fetch
Merge pull request #543 from akvo/feature/320

Feature/320
Merge branch 'develop' of github.com:akvo/akvo-flow into tests/524_clustering
[#524] (rm, ogl) Moved map cluster recentering calculations into the MapCluster class.
This avoids the need to expose the MapCluster internals since only the MapCluster itself needs to know how to calculate the new cluster centre.
Note: recentering calc tests are still broken and in progress.