Check filesize before uploading file to server and display alert if file is too big

[NicolasBuquet]

Check filesize before uploading file to server and display alert if file is too big

This test is present in the Android version of Element, but curiously not in the iOS version.

Pull Request Checklist

• [x] I read the contributing guide
• [ ] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [ ] Accessibility has been taken into account.
• [x] Pull request is based on the develop branch
• [x] Pull request contains a changelog file in ./changelog.d
• [x] You've made a self review of your PR
• [x] Pull request includes screenshots or videos of UI changes
• [x] Pull request includes a sign off

[NicolasBuquet]

@pixlwave Can you have a look at this PR ?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 12.32%. Comparing base (a5d4d1c) to head (f16203a). Report is 41 commits behind head on develop.

:exclamation: Current head f16203a differs from pull request most recent head 90f1122

Please upload reports for the commit 90f1122 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #7778 +/- ## =========================================== + Coverage 12.27% 12.32% +0.05% =========================================== Files 1651 1651 Lines 164167 163958 -209 Branches 67499 67364 -135 =========================================== + Hits 20148 20209 +61 + Misses 143346 143112 -234 + Partials 673 637 -36 ``` | [Flag](https://app.codecov.io/gh/element-hq/element-ios/pull/7778/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | Coverage Δ | | |---|---|---| | [uitests](https://app.codecov.io/gh/element-hq/element-ios/pull/7778/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq) | `55.67% <ø> (+0.65%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=element-hq#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.