Return full thumbail image in api

[dimasciput]

For https://github.com/qgis/QGIS-Django/issues/291

The new api will return :

{
    "total": 1,
    "next": null,
    "previous": null,
    "results": [
        {
            "resource_type": "Style",
            "uuid": "de3bd56f-1c49-43d7-97a8-ebc6008d6083",
            "name": "Cube",
            "creator": "creator",
            "upload_date": "2020-11-17T00:59:37.341000",
            "download_count": 1,
            "description": "This a cube",
            "file": "http://localhost:62202/media/styles/2023/qartoon.xml",
            "thumbnail": "http://localhost:62202/media/cache/be/7a/be7a1063302bd3a69734ee227bfa793b.jpg",
            "thumbnail_full": "http://localhost:62202/media/styles/2023/master_sword.jpg"
        }
    ]

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01 :tada:

Comparison is base (98041b0) 76.81% compared to head (16cfed7) 76.83%.

:exclamation: Your organization is not using the GitHub App Integration. As a result you may experience degraded service beginning May 15th. Please install the Github App Integration for your organization. Read more.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #295 +/- ## ========================================== + Coverage 76.81% 76.83% +0.01% ========================================== Files 128 128 Lines 4810 4813 +3 ========================================== + Hits 3695 3698 +3 Misses 1115 1115 ``` | [Impacted Files](https://app.codecov.io/gh/qgis/QGIS-Django/pull/295?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qgis) | Coverage Δ | | |---|---|---| | [qgis-app/api/serializers.py](https://app.codecov.io/gh/qgis/QGIS-Django/pull/295?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qgis#diff-cWdpcy1hcHAvYXBpL3NlcmlhbGl6ZXJzLnB5) | `91.17% <100.00%> (+0.26%)` | :arrow_up: | | [qgis-app/api/tests/test\_views.py](https://app.codecov.io/gh/qgis/QGIS-Django/pull/295?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qgis#diff-cWdpcy1hcHAvYXBpL3Rlc3RzL3Rlc3Rfdmlld3MucHk=) | `92.35% <100.00%> (+0.09%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.