Change crawler default settings around userAgent and mobileDevice

[benoit74]

Fix #291

Changes:

• Rollback previous changes around the presence of a default user-agent made in https://github.com/openzim/zimit/pull/229
• Set a default mobile device as "Pixel 2" so that it is more inline with the expectation that we are creating ZIMs for mobile devies (and Youtube videos have higher chance to work on all devices)
• Add a new --noMobileDevice to be able to completely remove this default value
• Remove trailing characters when retrieving Browsertrix Crawler version (unrelated to issue, just stumbled upon while debugging)
• Upgrade to Browsertrix Crawler 1.0.3 (not using 1.1.0-beta branch since it is less recent / seems to be upgraded less often)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 14.45%. Comparing base (5c71674) to head (728784d). Report is 2 commits behind head on zimit2.

Files	Patch %	Lines
src/zimit/zimit.py	0.00%	9 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## zimit2 #292 +/- ## ========================================== - Coverage 14.91% 14.45% -0.47% ========================================== Files 1 1 Lines 248 249 +1 Branches 35 35 ========================================== - Hits 37 36 -1 - Misses 211 213 +2 ```

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