Standardize modal visual presentation

[jessicaschilling]

Note: Need to wait until #1615 is merged to avoid throwing more conflicts on that PR's pile.

The need

Standardize visual presentation of modals throughout Web UI, including the following:

• Darker primary action text
• (Possibly) left-aligning modal content, though need to investigate how this works with code snippets etc
• Possibly lighter or lighter-weight text for checkbox and similar items
• Consider right-justifying buttons
• Updating storybook if necessary

Modals to address

Files screen:

• Import from IPFS
• New folder
• Share link
• Rename
• Delete
• (Pinning-related modals TK)

Peers screen:

• Add connection

Settings:

• Language
• Tutor mode
• (Pinning-related modals TK)