great component.

missing features:
- support maintenance windows, which enable the maintenance mode on a certain datetime and stops it at another datetime.
- easily select multiple applications, click on 'start or set maintenance mode' and start or configure it. 

What a great work ! I feel this component has a lot of potential to be reused in ecosystems with dozens (maybe hundreds) of applications.
I love the user interface, the look and feel is very clean and easy to use. The detail of having the timer ticking and showing the amount of time the app is under maintenance blowed my mind :)

What features I am missing ? 
- Statistics: to have the historical information of maintenance. Providing the ability to list all the maintenance time (aka unavailability/downtime) for each application/module, filtered by time. This will allow to answers like "how many maintenance events did we had last quarter?" , "how many times app XXX was under maintenance last month? " , "how much time app xxx and app yyy where unavailable last year ?". Don't forget the ability to export the report to excel/csv :)
- Globalization: In a globalized world we assume everyone is comfortable to use English as default language but better user experience can be provided if we had the ability to provide the maintenance message in several languages. The first approach can be to have a combobox to allow user to pick up the language. other ideias: for example, show country flags, and detect default language based on IP
- Maintenance text to allow html formatting to allow bold text, paragraphs (<b> <p>) or a <a href mailto:> tags to be embedded in the message
- Allow to Schedule maintenance by defining a maintenance window (maintenance start datetime and end datetime). To cover scenarios where we know that application XXX will be under maintenance next week  on Monday at 2AM GMT during 1 hour. This will allow operators to schedule the maintenance anytime avoiding them to be up at 2AM to switch on the maintenance mode and wait for a couple of hours to switch it off.

Seems to work pretty well in Reactive and Mobile apps.