Add a theme provider

[SjaakSchilperoort]

Resolves: #87

The following has been implemented:

• Add a theme provider that can be used by the apps to set a theme
• Add a useTheme hook to use the theme from the theme provider
• Add a useStyles hook to insert the current theme in a stylesheet factory method
• Turn the eslint rule no-unused-styles off, as this only works for static stylesheets

For all components:

• Replace all static imports of theme by the useTheme hook
• For all stylesheets that depend on theme, convert the static stylesheet to a factory method

Specifics:

• Create factory methods for LargeButtonStyles, so theme can be inserted
• Remove the FilterButton component, as this was based on static theming. The code is inlined in ObsIdentify's ObservationFilterBar.