Speaker list page requires horizontal scrolling on narrow devices (e.g: phones)

[chris48s]

Currently the speaker list uses a two-column layout. We hide the speaker photos on mobile, but this layout still requires horizontal scrolling to read on mobile devices. We should collapse into a single-column layout on mobile to make this more accessible for mobile users.

e.g:

before

[photo]       | description
speaker name  | text
---------------------------
[photo]       | description
speaker name  | text
...

after

[photo]
speaker name
description text
-----------------
[photo]
speaker name
description text
...