The project list table header is not sticky, but it should be.

### Steps to reproduce

1. Login as a user with permissions to see projects in the project list

2. Go to the project list

3. Scroll down until the header leaves the screen


### What is the buggy behavior?

* The header leaves the screen as you scroll.


Begin of the insertion<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/251152/content">End of the insertion Begin of the deletion <br>

<br>End of the deletion

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### What is the expected behavior?

* The header should be sticky and stay visible. Compare work package list.


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<img class="op-uc-image op-uc-image_inline" src="/api/v3/attachments/251148/content">

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**Browser**

* [x] Chrome
* [x] Firefox
* [x] Safari
* [x] Mobile Safari
* [ ] Other (please specify)

**Operating System**

* [ ] Windows
* [x] Mac OS
* [ ] Mobile iOS
* [ ] Mobile Android
* [ ] Linux (please specify distro)
* [ ] Chrome OS
* [ ] Other (please specify)