UI & visual tweaks for notifications

[cathydeng]

• [x] consolidate all user stuff in navbar to a dropdown menu if user is logged in
• [x] 'log in/sign up' in nav bar -> 'login'
• [ ] only require one password for signup
• [x] signup/login pane alignment & padding
• [ ] change '[create subscription]' & '[remove subscription]' to icons w/ state, w/ text upon hover. should prob also be button b/c it is an action, to differentiate between rss link next to it
• [ ] improve ui for removing subscriptions - remove button + modal confirmation
• [x] on manage subscriptions page, move explanatory text from side well to top of page. need to add some indication of what the alerts will be (just email?) & how they are triggered
• [x] add some icons on subscriptions page
• [x] 'alerts' -> 'subscriptions'

[reginafcompton]

Done (with the exception of the "one password for signup"). All these updates are live for NYC.