Implement /install-github-app flow

[svenefftinge]

We need to implement the flow for installing the Github app.

Currently it is:

1. https://github.com/apps/gitpod-io (click configure)
2. /install-github-app

3. Agree on github
4. confirmation

Designs

Install	Warning	Success	Error

See also design specs.

[svenefftinge]

There are currently three different outcomes:

<Dialog open={this.state.showDialog === 'cancel'}>
                    <DialogTitle>Cancel installation?</DialogTitle>
                    <DialogContent>
                        <DialogContentText>Canceling the installation here just prevents Gitpod from reacting to the events it receives from GitHub.
                        To fully undo the installation, please uninstall the <a href={`https://github.com/settings/installations/${this.installationId}`} target="_blank" rel="noopener noreferrer">Gitpod app in GitHub</a>.</DialogContentText>
                    </DialogContent>
                    <DialogActions>
                        <Button onClick={() => this.setState({ showDialog: 'none' })} variant="outlined" color="primary">Abort</Button>
                        <Button onClick={() => window.location.href = `https://github.com/settings/installations/${this.installationId}`} variant="outlined" color="secondary" autoFocus>Ok</Button>
                    </DialogActions>
                </Dialog>
                <Dialog open={this.state.showDialog === 'errorWhenReg'}>
                    <DialogTitle>Error while installing the app</DialogTitle>
                    <DialogContent>
                        <DialogContentText>Could not register the app with Gitpod - there seems to be a network issue. Please try again in a moment.</DialogContentText>
                    </DialogContent>
                    <DialogActions>
                        <Button onClick={() => this.setState({ showDialog: 'none' })} variant="outlined" color="secondary" autoFocus>Ok</Button>
                    </DialogActions>
                </Dialog>
                <Dialog open={this.state.showDialog === 'done'}>
                    <DialogTitle>Installation successful</DialogTitle>
                    <DialogContent>
                        <DialogContentText>The GitHub app was installed successfully. Have a look at the <a href="https://www.gitpod.io/docs/prebuilds/" target="_blank" rel="noopener">documentation</a> to find out how to configure it.</DialogContentText>
                    </DialogContent>
                    <DialogActions>
                        <Button onClick={() => window.location.href = thisUrl.asDashboard().toString()} variant="outlined" color="secondary" autoFocus>Ok</Button>
                    </DialogActions>
                </Dialog>

[gtsiolis]

Resolved in #3704.