Flash, flash & flash, the system is broken and we need some alternative

Current status:
most of the flash messages are not working.

including some weird configurations with:
```JavaScript
flash.type = 'Error!'
flash.class = 'alert-danger'
flash.messages = [{ msg: 'The invitation link is wrong or truncated. Key informations are missing.' }]

const buff = Buffer.from(JSON.stringify(flash))
const data = buff.toString('base64')

return res.redirect('/clans?flash=' + data)
```

there is an old flash implementation https://github.com/jaredhanson/connect-flash, but this is 10 years old and actually not used in the code, we start it with:

```JavaScript
#AppKernel
this.expressApp.use(flash())
this.expressApp.use((req, res, next) => {
res.locals.message = req.flash()
next()
})
```
but never written or used in the templates....

To stop this madness, we need a proper session-flash implementation and stop using multiple systems or structures.
No reddirest with "?flash=knhdsfapoinbgoiua", no custom made objects and template mixins.

Base Acceptance criteria:
- if the user destroys a clan or makes any mutating action, a flash message should be saved in the session
- on rendering a site, a flash-message should pop up

non-goals:
- using flash messages to show validation errors

something like