[Tabs] Update so that tabs keep equal widths

[sosaucily]

Without this, the content in the tab label will too easily cause the tabs to have uneven width.

• I have followed (at least) the PR section of the contributing guide.

[mui-pr-bot]

Details of bundle changes.

Comparing: 2dedcc1...8306ef3

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	350,243	350,255	90,024	90,026
@material-ui/core/Paper	0.00%	0.00%	67,867	67,867	19,823	19,823
@material-ui/core/Paper.esm	0.00%	0.00%	60,198	60,198	18,568	18,568
@material-ui/core/Popper	0.00%	0.00%	30,463	30,463	10,527	10,527
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	17,384	17,384	5,729	5,729
@material-ui/core/useMediaQuery	0.00%	0.00%	2,469	2,469	1,041	1,041
@material-ui/lab	0.00%	0.00%	148,279	148,279	43,580	43,580
@material-ui/styles	0.00%	0.00%	53,099	53,099	15,433	15,433
@material-ui/system	0.00%	0.00%	17,136	17,136	4,525	4,525
Button	0.00%	0.00%	87,947	87,947	26,064	26,064
Modal	0.00%	0.00%	82,055	82,055	24,563	24,563
colorManipulator	0.00%	0.00%	3,232	3,232	1,299	1,299
docs.landing	0.00%	0.00%	49,958	49,958	10,837	10,837
docs.main	0.00%	0.00%	645,407	645,407	200,517	200,517
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	+0.01% 🔺	298,542	298,554	82,758	82,765

Generated by 🚫 dangerJS against 8306ef3

[sosaucily]

This can be tested on the demos page, by shrinking the page a bit (like width 800px) and noticing that the widths of the full-width tabs are no longer equal. Then add this css rule and see how it's fixed.

[oliviertassinari]

@sosaucily It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!

[sosaucily]

Love the project, happy to help! @oliviertassinari