message: change the slack username input style
diff: <div class="row">
  <div class="col s12 m6">
  <div class="input-field">
  - <label for="githubUrl" id="slack-label">C4SG Slack Username</label>
  - <input materialize="tooltip" [materializeParams]="[{html: true}]" [maxlength]="25" class="theme-input tootipped" id="chatUsername" type="text" formControlName="chatUsername" data-position="bottom" data-delay="50" data-tooltip="This is your username in Slack team of Code for Social Good.<br />People could direct message you by accessing the link 'https://code4socialgood.slack.com/messages/@<your_slack_username>'.<br />To join the team, click Chat under Community in the header.<br />The Slack team encourages volunteers and nonprofits communicate with each other to build a strong volunteering community.">
  + <label materialize="tooltip" [materializeParams]="[{html: true}]" data-position="bottom" data-delay="50" for="githubUrl" id="slack-label"
  + data-tooltip="This is your username in Slack team of Code for Social Good.<br />People could direct message you by accessing the link 'https://code4socialgood.slack.com/messages/@<your_slack_username>'.<br />To join the team, click Chat under Community in the header.<br />The Slack team encourages volunteers and nonprofits communicate with each other to build a strong volunteering community.">C4SG Slack Username</label>
  + <input [maxlength]="25" class="theme-input tootipped" id="chatUsername" type="text" formControlName="chatUsername">
  </div>
  </div>
  <div *ngIf="isOrganization">
  