Fixed XSS vulnerability in folder name

[codecov[bot]]

Codecov Report

Base: 77.53% // Head: 77.55% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (d3cea44) compared to base (3b3b3b7). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #245 +/- ## ========================================== + Coverage 77.53% 77.55% +0.01% ========================================== Files 24 24 Lines 2195 2196 +1 ========================================== + Hits 1702 1703 +1 Misses 493 493 ``` | [Impacted Files](https://codecov.io/gh/modoboa/modoboa-webmail/pull/245?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa) | Coverage Δ | | |---|---|---| | [modoboa\_webmail/templatetags/webmail\_tags.py](https://codecov.io/gh/modoboa/modoboa-webmail/pull/245?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa#diff-bW9kb2JvYV93ZWJtYWlsL3RlbXBsYXRldGFncy93ZWJtYWlsX3RhZ3MucHk=) | `74.48% <100.00%> (+0.26%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=modoboa)

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